In order to reduce harmful gas components discharged from spark-ignition internal combustion engines that are mounted on vehicles, or improve fuel-efficiency of the engines, various gasoline engines of an in-cylinder injection type, in which fuel is injected directly into a combustion chamber, have recent been proposed in place of conventional manifold-injection engines.
In an in-cylinder injection gasoline engine, an air-fuel mixture with an air-fuel ratio near a stoichiometric air-fuel ratio is generated around a spark plug at the time of ignition by, for example, injecting fuel from a fuel injection valve into a cavity formed in the top portion of a piston. Thus, the fuel can be ignited with use of a generally lean air-fuel ratio, so that the deliveries of CO and HC are reduced, and the fuel-efficiency during idle operation or low-load drive operation can be improved considerably.
In the gasoline engine of this type, moreover, the injection mode is switched between a compression-stroke injection mode and an intake-stroke injection mode, depending on the operating state of the engine, that is, engine loads. When the engine is in low-load operation, the fuel can be injected mainly during a compression stroke so that an air-fuel mixture with an air-fuel ratio near the stoichiometric air-fuel ratio can be locally formed around the spark plug or in the cavity. Thus, the fuel can be ignited satisfactorily even with use of a lean overall air-fuel ratio. When the engine is in medium- or high-load operation, on the other hand, the fuel can be injected during an intake stroke so that an air-fuel mixture with a uniform air-fuel ratio can be formed in the combustion chamber. As in the case of a manifold-injection gasoline engine, therefore, the fuel can be burned to secure a required output for acceleration or high-speed drive.
In a conventional manifold-injection gasoline engine, the inflammable air-fuel ratio region (lean burn region) of an air-fuel mixture supplied to the engine is so narrow that a substantially constant output torque can be obtained within the inflammable air-fuel ratio region if a volumetric efficiency Ev is fixed. Thus, the volumetric efficiency Ev and the output torque are substantially in unique relation. In the conventional manifold-injection gasoline engine having this property, engine control parameter values, such as a target air-fuel ratio, target ignition timing, etc., are set by using the volumetric efficiency Ev that is obtained from an airflow sensor output, and the operation of the engine is controlled in accordance with these control parameter values.
In the control of the in-cylinder injection gasoline engine in the compression-stroke injection mode, however, the fuel is injected into the cavity in the top portion of the piston to effect the stratified-charge lean combustion as a whole. Accordingly, normal combustion can be achieved if an inflammable air-fuel mixture exists only around the spark plug. When compared with the manifold-injection gasoline engine, therefore, this engine enjoys a much wider inflammable air-fuel ratio range in terms of the overall air-fuel ratio. Thus, in the control of the in-cylinder injection gasoline engine in the compression-stroke injection mode, operation can be carried out in a wider air-fuel ratio range from a very lean air-fuel ratio (e.g., air-fuel ratio of 50) to a rich inflammable-limit air-fuel ratio (e.g., air-fuel ratio of 20). Although the value of the volumetric efficiency is fixed, therefore, the output torque varies considerably as the target air-fuel ratio is changed, so that the output torque can be obtained substantially in proportion to the quantity of fuel supply. This implies that it is difficult to use the aforesaid volumetric efficiency Ev to set the engine control parameter values, such as the target air-fuel ratio and the target ignition timing, in the compression-stroke injection mode of the in-cylinder injection gasoline engine.
In consideration of these circumstances, the inventor hereof has proposed to use an in-cylinder effective pressure Pe in place of the volumetric efficiency Ev, as a parameter representative of the engine output, in setting the engine control parameter values, such as the target air-fuel ratio and the target ignition timing, in the compression-stroke injection mode of the in-cylinder injection gasoline engine, or in discriminating mode change between the compression-stroke injection mode and the intake-stroke injection mode. More specifically, the target in-cylinder effective pressure (target load value) Pe correlated with a driver's desired engine output is obtained from an accelerator opening (throttle opening) and engine speed (rotational speed), and the fuel supply quantity (target air-fuel ratio), ignition timing, etc. are set in accordance with this target value Pe.